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8. How Aspell Works

The magic behind my spell checker comes from merging Lawrence Philips excellent metaphone algorithm and Ispell's near miss strategy which is inserting a space or hyphen, interchanging two adjacent letters, changing one letter, deleting a letter, or adding a letter.

The process goes something like this.

1. Convert the misspelled word to its soundslike equivalent (its metaphone for English words).
2. Find all words that have a soundslike within one or two edit distances from the original words soundslike. The edit distance is the total number of deletions, insertions, exchanges, or adjacent swaps needed to make one string equivalent to the other. When set to only look for soundslikes within one edit distance it tries all possible soundslike combinations and check if each one is in the dictionary. When set to find all soundslike within two edit distance it scans through the entire dictionary and quickly scores each soundslike. The scoring is quick because it will give up if the two soundslikes are more than two edit distances apart.
3. Find misspelled words that have a correctly spelled replacement by the same criteria of step number 2 and 3. That is the misspelled word in the word pair (such as teh -> the) would appear in the suggestions list as if it was a correct spelling.
4. Score the result list and return the words with the lowest score. The score is roughly the weighed average of the weighed edit distance of the word to the misspelled word and the soundslike equivalent of the two words. The weighted edit distance is like the edit distance except that the various edits have weights attached to them.
5. Replace the misspelled words that have correctly spelled replacements with their replacements and remove any duplicates that might arise because of this.

Please note that the soundslike equivalent is a rough approximation of how the words sounds. It is not the phoneme of the word by any means. For more details about exactly how each step is performed please see the file suggest.cc. For more information on the metaphone algorithm please see the data file english_phonet.dat.

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